1. Field of the Invention
The present invention relates to a buffer amplifier, and more particularly, to a buffer amplifier including a pair of differential N-type metal-oxide semiconductor field-effect transistors (NMOSFETs) and a pair of differential P-type metal-oxide semiconductor field-effect transistors (PMOSFETs) amplifying differential input signals and biased through a feedback resistor so as to operate the pairs of differential NMOSFETs and PMOSFETs in a saturation region.
2. Description of the Related Art
In general, wireless communication systems include various blocks to amplify and filter signals, convert frequencies, and the like. There are buffers of the various blocks receiving and amplifying local oscillator signals generated by local oscillators and outputting the amplified signals to mixers so as to reduce variations in oscillator frequencies caused by effects of loads.
FIG. 1A is a circuit diagram of a differential common-source buffer amplifier using a conventional inductor load.
As shown in FIG. 1A, in a case where inductors L1 and L2 are loads, the inductors L1 and L2 are advantageous in terms of a power consumed for driving a capacitance load. However, inductors occupy wider areas than other elements in an integrated circuit and thus increase the size of the integrated circuit. Also, in a case where the inductors are directly used as loads, a quality factor may be low, and an amplifier sensitively operates with respect to a parasitic capacitance component.
FIG. 1B is a circuit diagram of a conventional differential common-source buffer amplifier using resistor loads.
As shown in FIG. 1B, in a case where resistors RL1 and RL2 are used as loads, inductors are not directly used as loads. Thus, a size of an integrated circuit can be reduced. However, a bias current Ibias is increased to increase a transconductance gm of an amplifier node including differential transistors MN1 and MN2. Thus, power consumption is increased, and a voltage drops in the resistors RL1 and RL2. As a result, it is difficult to increase the amplitude of an oscillator signal. Also, a signal distortion occurs due to a limited Slew rate.
FIG. 1C is a circuit diagram of a conventional complementary metal-oxide semiconductor (CMOS) inverter type buffer amplifier. As shown in FIG. 1C, in a case where a PMOSFET MP1 and an NMOSFET MN1 constitute a CMOS inverter type buffer amplifier, a single signal is processed. Thus, the CMOS inverter type buffer amplifier is more disadvantageous in terms of removing a common mode of a power or a signal than the differential common-source buffer amplifier processing differential signals. In particular, a greater noise than the signal may exist due to swaying of the power or a harmonic component. Also, it is difficult to design a bias circuit.